1. Field of the Invention
The present invention is directed to the field of producing biodiesel from a lipid. In particular, the present invention is directed to a process using a gaseous alcohol to convert a lipid to biodiesel and strip impurities from the lipid and/or biodiesel product.
2. Description of the Related Technology
There are many factors that have led to increased research into alternative fuels and renewable energy. Some of these factors include rising prices of crude petroleum, concerns about carbon dioxide emissions, worsening air quality by emissions of sulfur oxides, particle matter and other gases, as well as security of domestic energy supply coupled with limited long-term supplies of petroleum.
Biodiesel is a promising renewable fuel, which contains mostly fatty acid alkyl esters. Biodiesel is typically produced chemically by reacting plant or animal derived lipids with an alcohol. The majority of biodiesels are produced by reacting lipids with methanol to produce fatty acid methyl esters (FAME). Currently, most of the lipids for biodiesel production are refined lipids that have low free fatty acid (FFA) concentrations, such as soybean oil (in USA), rapeseed oil (in Europe), and palm oil (in Asia). However, these refined lipids are agricultural crops that are relatively high-cost, because their production requires significant fertilizer and chemical inputs.
Apostolakou et al. (Techno-economic analysis of a biodiesel production process from vegetable oils, Fuel Processing Technology, vol. 90, pages 1023-1031, 2009) and Haas (Improving the economics of biodiesel production through the use of low value lipids as feedstocks: vegetable oil soapstock, Fuel Processing Technology, vol. 86, pages 1087-1096, 2005) showed that the cost of raw materials can be 75%-90% of the cost of manufacturing biodiesel. Marchetti et al. (Techno-economic study of different alternatives for biodiesel production, Fuel Processing Technology, vol. 89, pages 740-748, 2008) discussed three scenarios for producing biodiesel from lipids containing 5% FFA. All three scenarios are profitable with raw materials being more than 80% of the manufacturing costs. Zhang et al. (Biodiesel production from waste cooking oil: 1. Process design and technological assessment, Bioresource Technology, vol. 89, pages 1-16 2003; and Biodiesel production from waste cooking oil: 2. Economic assessment and sensitivity analysis, Bioresource Technology, vol. 90, pages 229-240, 2003) compared four biodiesel production processes and found that the production of biodiesel from refined lipids had the lowest capital expense and highest cost of raw materials, while the processes converting waste lipids to biodiesel required higher costs on more methanol and larger distillation columns to recover unreacted methanol, though the cost on waste lipids is much lower. Because the feedstock is a major fraction of the biodiesel manufacturing costs, the process using low cost waste lipids will likely show much greater economic feasibility.
The costs of lipids are related to their FFA content. Edible lipids have low FFA content and command relatively high prices. Inedible lipids tend to be high in FFA and have low prices. The high-FFA lipids are mostly waste products and have limited commercial value, while low-FFA lipids tend to be viable food sources. For example, soybean oil currently sells for about $3.52 per gallon, and yellow grease (filtered and dewatered waste cooking oil with FFA content below 15%) sells for $2.19 per gallon. Trap grease is a potential source of high-FFA lipids, because wastewater utilities charge $0.06 or more per gallon to dispose of trap grease. Lipids separated from trap grease, which may be 2%-10% of the trap grease, can have over 95% FFA. Producing biodiesel from high-FFA lipids entails low feedstock costs and is less prone to controversies associated with producing fuels from food-grade lipids (M. Canakci, The potential of restaurant waste lipids as biodiesel feedstocks, Bioresource Technology, vol. 98, pages 183-190, 2007; and K. S. Tyson, DOE analysis of fuels and coproducts from lipids, Fuel Processing Technology, vol. 86, pages 1127-1136, 2005).
The high cost of the raw materials for biodiesel is one of the major reasons that biodiesel is not an ideal solution for the energy demand in the United States. Van Gerpen (Biodiesel processing and production, Fuel Processing Technology, vol. 86, pages 1097-1107, 2005) noted that only about 14% of current diesel demand can be replaced by biodiesel produced from crop-based lipids. If the cost of biodiesel production can be lowered by using low cost lipids, the production capacity of biodiesel may be expanded and biodiesel may become more widely used. The available feedstock for biodiesel production may double if waste greases are widely used for biodiesel production.
It has been proposed that high-FFA lipids such as waste lipid feedstocks can provide significant cost reduction and production capacity for biodiesel (Tyson et al., Biomass Oil Analysis: Research Needs and Recommendations, NREL, Golden Colo., 2004). There are several technologies available for converting high-FFA lipids to FAME. Acid-catalyzed esterification technology is effective for lipids over a large range of FFA concentrations. This technology is often used for pretreatment of lipids prior to base-catalyzed transesterification in a two-step process. A significant disadvantage of acid-catalyzed esterification is slower reactions. There are several ways to increase acid-catalyzed esterification reaction rates, including increasing temperature, increasing catalyst concentration, and removing by-product water.
For low FFA lipids (containing 1%-10% FFA), a two-step process including low-temperature acid-catalyzed esterification followed by base catalyzed transesterification is commonly used for converting the lipid to biodiesel. For lipids containing more than 50% FFA, a process with multiple moderate-pressure reactors with intermediate removal of water are used effectively for producing biodiesel (W. W. Berry, B. J. Ratigan, Process of making alkyl esters of free fatty acids, Philadelphia Fry-o-Diesel Inc., US, 2010). Multiple, identical reactors with intermediate water removal will increase reaction speed and conversion. But the process with multiple reactors also increases the capital and operating costs significantly. In addition, to achieve acceptable reaction speed, temperatures above the boiling point of methanol are often used, which requires elevated pressure to maintain methanol in the liquid phase. For example, Van Gerpen reports using 240° C. and 90 bar for such a process (Biodiesel processing and production, Fuel Processing Technology, vol. 86, pages 1097-1107, 2005) and Berry and Ratigan report 115° C. and 5.4 bar in a similar process (Process of making alkyl esters of free fatty acids, Philadelphia Fry-o-Diesel Inc., US, 2010). The heating and pressurizing of the reactor will also increase the operation costs.
Kocsisova et al. (High-temperature esterification of fatty acids with methanol at ambient pressure, European Journal of Lipid Science and Technology, vol. 107, pages 87-92, 2005) discloses a kinetic study of the acid-catalyzed esterification of free fatty acids with methanol at elevated temperatures above the boiling point of methanol, at ambient pressure, and at continual flow of liquid methanol into the reaction mixture. Under these conditions, the esterification reaction follows the rate equation for reactions of the first order. At temperatures that are 50-60° C. higher than the boiling point of methanol, the reaction rate is two to three times higher than at the temperatures close to the boiling point of methanol. Beside temperature, the reaction rate depends also on the flow rate of methanol and on the concentration of the catalyst. A high local molar excess of methanol in the input site with respect to FFA and effective removal of water from the reaction mixture also increase the reaction rate. High conversion of FFA to methyl esters (above 99%) with low residual acidity of the product (acid value around 2-3 mg KOH/g) is achieved in several tens of minutes at a low total molar ratio of methanol/FFA of around 3:1 to 4:1.
U.S. Pat. No. 8,603,198 discloses a method for producing fatty acid alkyl esters from lipids through transesterification and/or esterification using a flow-through cavitation device for generating cavitation bubbles in a fluidic reaction medium. The fluidic medium is passed through sequential compartments in the cavitation device having varying diameters and inner surface features to create localized reductions in fluid pressure thus vaporizing volatile alcohols in the medium to create volatile alcohol-filled bubbles, which provide an increased surface area and optimized conditions for the transesterification and/or esterification to occur at the gas-liquid interface. The method can produce fatty acid alkyl esters and a glycerol, with the former being used in biodiesel.
There is a need of a low cost and efficient process to convert lipids with high FFA concentration such as waste lipids to biodiesel. The present invention provides a method based on passing bubbles with an alcohol vapor through lipids with high concentrations of free fatty acids. The present invention has low energy cost and low feedstock cost, which results in producing biodiesel at a much lower cost. The low cost biodiesel will likely lead to wider applications for the biodiesel.